Apple does not support syncing/sharing folders inside iCloud Drive between users, for example in a family. Is there a third-party solution that provides this functionality?
The quirk here is that it needs to be aware of macOS's new optimized storage feature. This feature will remove a file "foo.txt" from iCloud Drive, store it in the cloud and only leave a ".foo.txt.icloud" empty placeholder file behind. A plain file synching solution like Dropbox would not understand these, and inadvertently delete files. Therefore the solution needs to be aware of iCloud Drive's characteristics.
Additionally the synced folder needs to be specifically inside iCloud Drive, so that the synced files are available for use through various iCloud Drive-compatible interfaces.